I can fully recommend SmartKnitKIDS seamless undies and seamless socks.
In September DD3 started school and was going wild each morning with her seams on socks. Even turning them inside out wasn't good enough. After having to carry her into school without shoes or socks, I ordered SmartKnitKIDS socks from Sensory Smart Store. Socks stopped being an issue. We still had to sew down all the name tags in uniform, but so far so good.
The trouble was, there were only 2 pairs of knickers that DD3 would wear. Every morning we battled with the knicker selection. I'd get out 8 pairs and she'd go through them, trying them on, ripping them off. Even the pairs she would tolerate were only put on after threats to leave her behind.
I thought that after settling in school, it would improve, but even in the holidays, she's preferred to be naked.
So, yesterday I ordered some SmartKnitKIDS seamless knickers. The FB page of the shop said 'we're not processing orders until Monday 6th January, but if you have any sensory emergencies, let me know and I'll see what I can do.' I emailed the lady and just said 'look, it probably wouldn't get here anyway, but if you could possibly dispatch today, I'd be grateful.' She emailed back and said 'ok, I'll dispatch shortly'.
Today, they arrived. DD3 tried a pair on and exclaimed 'Oh their so soft, and this bit isn't annoying, where there used to be a line! Can I wear them all day??'
They're expensive, but so worth it.